The invention relates to a pump jack for pumping liquids, especially oil from wells.
One conventional type of pump jack for pumping oil, water or other liquids from a well comprises a rocker arm pivotally mounted intermediate its ends on a support member. On one limb (hereinafter referred to as the sucker-rod limb) of this rocker arm are fixed sucker rod attachment means which are connected to a sucker rod which descends into the well and is connected to the piston of a reciprocatory pump mounted within the well, at the bottom or at some other level from which the liquid is to be pumped. Usually, a counterweight is mounted upon the opposed limb of the rocker arm (hereinafter referred to as the "drive limb") to counterbalance the greater weight of the sucker rod and piston. To pivot the rocker arm, and thus to reciprocate the sucker rod vertically within the well, the upper end of a crank is fixed to the drive limb of the rocker arm. The lower end of this crank is connected to a rotating arm fixedly mounted on a rotating drive shaft positioned below the drive limb of the rocker arm. The drive shaft is rotated by a gearbox from any conventional type of motor, this motor usually either being an electric motor or an internal combustion engine. The rotation of the drive shaft causes the sucker rod to reciprocate vertically; the motion of the sucker rod is substantially simple harmonic motion, subject only to minor, second-order deviations due to the displacement of the crank from the vertical during the rotation of the drive shaft. Thus, approximately half-way through its upstroke, the sucker rod is traveling at its maximum velocity and from this point there is applied to the sucker rod a progressively increasing downward acceleration until the sucker rod finally halts at the end of its upstroke. This same downward acceleration is continued into the first part of the downstroke, but decreases progressively until, approximately half-way through the downstroke, no acceleration is being applied, although the sucker rod is moving downwardly at its maximum velocity. For the remaining half of the downstroke, there is applied to the sucker rod a steadily increasing upward acceleration until the sucker rod reaches the end of its downstroke, whereupon this upward acceleration is continued but at a steadily decreasing rate until the upward acceleration ceases approximately half way through the next upstroke. The maximum accelerations imposed upon the sucker rod are considerable; for example, in a typical conventional pump jack having a stroke of three feet (0.91 m) and a five-second pumping cycle (a pumping cycle comprising one upstroke and one downstroke), the maximum acceleration upon the sucker rod is approximately 2.4 feet per second.sup.2 (0.73 m.sec..sup.-2).
The loads imposed upon the sucker rod of an oil well pump jack are considerable. During the upstroke in a typical oil well, the weight of the sucker rod and the oil being lifted therewith amounts to about 1.6 pounds per foot (2.38 kg/m) of well depth and thus about eight thousand pounds (3629 kg) in a 5,000 ft. (1524 m.) well and many oils wells are considerably deeper. When a conventional rocker arm oil well pump jack is in use, it is obvious to even the casual observer that very large shock loadings are being placed upon the sucker rod as the sucker rod reverses its motion at the end of the upward and downward stroke; often the frame supporting the rocker arm can be seen to flex and vibrate, especially as the sucker rod begins its upward stroke. I have concluded that these large shock loadings upon the sucker rod arise because there is a large difference between the upwardly directed force which is needed to stop the downward stroke of the sucker rod and that necessary to cause the sucker rod to begin its upward stroke. During its downward stroke, the sucker rod and the piston connected thereto do not have to support the weight of the column of oil within the well (obviously, the well is provided with means to prevent the column of oil flowing back down the well as the sucker rod and piston descend). Thus, to stop the downward stroke of the sucker rod, the pump jack need only impose on the sucker rod an upwardly directed force about equal to the weight of the sucker rod and piston less the weight of any counterweights used. However, during the upward stroke of the sucker rod, not only must the sucker rod and piston be lifted, but also the column of oil within the well, thus, at the beginning of the upward stroke of the sucker rod the pump jack must impose upon the sucker rod an upwardly directed force at least about equal to the weight of the sucker rod, piston and the column of oil in the well, again less the counterweights employed. The column of oil in a typical 5,000 ft. (1524 m.) well weighs about 3,000 pounds (1361 kg.) and thus at the beginning of each upward stroke this weight is instantaneously imposed upon the sucker rod, resulting in a massive shock loading thereon. This shock loading is made worse by the fact that, since a conventional pump jack sucker rod undergoes substantially simple harmonic motion, the acceleration is at a maximum at the extremities of the motion where the shock loading occurs. These repeated shock loadings upon the sucker rod tend eventually to cause fractures thereof, leaving a considerable length of broken sucker rod in the well. To retrieve the broken sucker rod, a crew must be employed to fish the broken rod out through the surrounding casing, a procedure which involves considerable expense and a lengthy interruption of production from the well, since pumping of oil therefrom cannot be resumed until the broken sucker rod has been removed and replaced with a new one.
Also, a considerable proportion of the cost of a conventional oil well pump is accounted for by the gearbox necessary to reduce the speed of rotation of the prime mover driving the pump jack to the speed of rotation to the drive shaft bearing the rotating arm. Largely because of the considerable shocks imposed upon the whole apparatus by the sucker rod, such gearboxes must be made very robust and are correspondingly expensive to produce.
It will be appreciated that the problems mentioned above are not confined to oil wells, but may be experienced in other wells, such as water wells, which pump liquid to the surface in substantially the same manner as an oil well.
Numerous proposals have been made for reducing the shocks experienced by sucker rods at the end of their strokes, and some of these proposals also reduce the cost and complexity of the gearbox required in the pump jack. For example, U.S. Pat. No. 1,592,391 issued July 13, 1926 to Stevenson proposes a pump jack in which, as in a conventional pump jack, a crank is pivotally mounted at its upper end on the drive limb of a rocker arm. However, in Stevenson's pump the lower end of the rocker arm is rotatably and eccentrically mounted on a driven sprocket. This driven sprocket engages a chain, which also passes around a smaller driving sprocket mounted upon the base of the pump jack and is driven by any convenient type of prime mover. Although Stevenson's pump may reduce the cost of the necessary gearbox, since the final reduction in drive speed is accomplished by the gearing between the small driving sprocket and the large driven sprocket, calculations presented below show that it has only a smal effect in reducing the acceleration at the beginning of the upstroke of the pump and thus little effect in reducing the shock loading on the sucker rod at the beginning of the upstroke.
A further prior art proposal, set forth in U.S. Pat. No. 2,488,124 issued Nov. 15, 1949 to Hawley, again involves the use of a small driving sprocket mounted on the base of a pump jack, and a chain engaging this driving sprocket and a much larger driven sprocket. In Hawley's pump, however, the large driven sprocket is eccentrically mounted directly on the rocker arm, the rocker arm being in the form of a split beams so that the driven sprocket can be accomodated between the two parts of the sucker rod. Hawley's pump does reduce the complexity of the gearbox required to drive the pump jack and, as will be shown by calculations below, does help to reduce the acceleration imposed upon the sucker rod at the beginning of the upstroke and thus the shock loading imposed upon the sucker rod.
I have now devised a pump jack which, like the Stevenson and Hawley pump jacks described above, makes use of an eccentrically mounted circular driven member. However, my pump jack has certain features not found in these prior art proposals and calculation shows that it is more successful than either the Hawley or Stevenson proposals in reducing the acceleration imposed upon the sucker rod at the beginning of the upstroke, and thus in reducing the shock loading upon the sucker rod.